The regulating stage of high-pressure steam turbines typically features a relatively small blade height, which leads to high secondary flow losses. Various methods have been proposed in the literature to mitigate these losses and improve stage performance, including the use of endwall contouring. This study investigates endwall contouring using Widosinski curve for a regulating stage. Subsequently the Widosinski curve is parameterized to optimize the endwall contouring. By employing a multi-objective genetic algorithm, an optimized endwall contouring curve is obtained. Numerical results show a significant reduction in secondary flow losses for both the stator and rotor. Compared to the original flat endwall contouring, for the stator, the secondary flow loss is reduced by 20.1%, 39.6%, and 68.3% for the 100%, 33%, and 20% working loads respectively. For the rotor, it is correspondingly reduced by 57.3%, 38.2%, and 29.2%. Moreover, the total-total efficiency increases by 2.145%, 4.096%, and 5.421% for the aforementioned three load conditions. Through an analysis of the internal flow field, it is observed that the endwall contouring effectively suppresses vortex intensity in the channel and minimizes expansion along the span of the regulating stage. As a result, secondary flow losses are reduced. The findings of this study hold practical implications for the optimization of real steam turbines.